A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In the ongoing efforts for attaining higher resolutions at smaller imaging dimensions in lithographic systems, projection systems are used that have numerical apertures that are well above 0.8. These systems tend to be very bulky and wide in dimensions, in particular near the lower surface of the system where the radiation (light) exits the projection system for illuminating a substrate. Furthermore, these high numerical apertures have a working distance that is usually very small (only a few millimeters), which causes the projection system to be placed very close to the wafer surface. As a result of this, in practical setups, there is very little room provided for arrangements that are operative in the area between the lower surface of the projection system and the substrate. One of these arrangements that are operative in the above indicated area is an gas shower system that is present to condition the “gas” (which is usually a very fine conditioned composition of gases) in this area. This conditioning, among others, is necessary for defining a stable gas environment so that interferometric measurement beams are unhindered by refractive index fluctuations. This is necessary for obtaining reliable (sub) nanometer measurements of the position of the substrate in relation to the projection system, so that a pattern is reliably imaged at a predefined position of the substrate.
However, the above indicated developments of bulkier projection systems and closer working distances thus make it difficult to position the gas shower system in such a way that this area as a whole is sufficiently reached. In particular, due to the lower working distance and bulkier projection system, there is virtually not enough room to place an gas shower system in such a way that the gas flow is sufficiently able to condition the volume located below a lower plane of the machine setup above the wafer table. Generally, such a lower plane may be formed by the lowest plane of the projection system, which could be the exit plane of the lower lens. Otherwise this plane could be formed by the Z-mirror that is used in determining the z-height of the substrate to be illuminated.